1. Technical Field
The present invention relates to an active-matrix-type light-emitting device and a pixel driving method for the active-matrix-type light-emitting device. In particular, the invention relates to a technique for effectively preventing black float (a phenomenon in which an unnecessary current flows even at the time of black display and a light-emitting element emits a small amount of light to thereby increase a black level, and as a result, the contrast decreases) at the time of black display of a pixel having a self-luminous element, such as an electroluminescent (EL) element.
2. Related Art
In recent years, an electroluminescent (EL) element having features, such as a high efficiency, a small film thickness, a light weight, and a low dependency on viewing angle, has been drawing attention and a display using the EL element is under active development. The EL element is a self-luminous element that emits light via application of an electric field to a fluorescent compound and is classified into one of two types, namely, an inorganic EL element using an inorganic compound, such as zinc sulfide, as a light-emitting material layer or an organic EL element using an organic compound, such as diamines, as a light-emitting material layer.
Since the organic EL element is advantageous in that obtaining different colors is easy and the organic EL element can operate at a low-voltage DC current that is much lower than that required for the inorganic EL element, application of the organic EL element to, for example, a display device of a portable terminal is expected in the near future.
The organic EL element is configured such that organic molecules forming an emission center are excited by injecting holes into a light-emitting material layer through a hole injection electrode and injecting electrons into the light-emitting material layer through an electron injection electrode and then causing the injected holes and electrons to be recombined, and fluorescent light is emitted when the excited organic molecules return to a ground states. Accordingly, an emission color of the organic EL element can be changed by selecting a fluorescent material used to form the light-emitting material layer.
In the organic EL element, electric charges are accumulated when a positive voltage is applied to a transparent electrode, which is an anode, and a negative voltage is applied to a metal electrode, which is a cathode, and a current starts to flow when a voltage value exceeds a barrier voltage unique to an element. Then, emission having an intensity that is approximately proportional to the DC current value occurs. That is, it can be said that the organic EL element is a current driving type self-luminous element like a laser diode, a light-emitting diode, and so on.
Methods of driving an organic EL display device are broadly classified into a passive matrix method and an active matrix method. In the case of the passive matrix driving method, the number of display pixels is limited and there are limitations in terms of lifetime and power consumption. For this reason, in many cases, an active-matrix-type driving method that is advantageous in realizing a display, for which a large area and high precision are requested, is used as a method of driving an organic EL display device. Accordingly, a display using the active-matrix-type driving method is under active development.
In the display device using the active-matrix-type driving method, a polysilicon thin-film transistor (polysilicon TFT) serving as an emission control transistor is formed for each of a plurality of electrodes in order to independently drive an organic EL element formed on each electrode, the electrodes of the polysilicon thin-film transistors being patterned in a dot matrix arrangement. In addition, the polysilicon TFT may also be used as a driving transistor for driving an organic EL element or a control transistor for controlling an operation related to data writing.
In the following description, the polysilicon TFT may be simply referred to as “TFT”. In the case of the “TFT”, a material thereof is not limited to polysilicon. For example, the material may be amorphous silicon.
An emission gray scale of an organic EL element is greatly affected by the characteristics of a TFT. In JP-A-2006-17966, considering that electric charges stored in a holding capacitor fluctuate due a leak current (optical leak current) generated in a TFT driven through a scanning line when light is illuminated, the fluctuation of the electric charges is suppressed by inserting a diode.
In JP-A-2006-17966, the optical leak current of the TFT is an issue. However, the leak current generated in the TFT also includes a leak current (dark current) generated when the TFT is in an OFF state and a leak current generated due to a circuit operation. Accordingly, it is necessary to examine the leak currents described above in a comprehensive way.
The inventor of the invention has studied the occurrence of a phenomenon (black float) in which a small but unnecessary current flows at the time of black display (that is, a state in which a current from a driving transistor is not supplied even though an emission control transistor is in an ON state, and as a result, a light-emitting element maintains a non-emission state) of an active-matrix-type light-emitting device, the light-emitting element emits light to thereby raise a black level, and accordingly, the contrast decreases and has examined the cause of the phenomenon in a comprehensive way.
It was determined that an instantaneous and large leak current, which is generated due to a circuit operation, is strongly related to generation of black float.
That is, when shifting an emission control transistor from an OFF state to an ON state by changing the electric potential of a scanning line, a changed component of the electric potential of the scanning line leaks to a light-emitting element through a parasitic capacitance between a gate and a source of the emission control transistor. As a result, a large amount of current flows instantaneously. This current is referred to as a “coupling current” In the following description. The “coupling current” is a current resulting from a transitional pulse that is coupled to a light-emitting element through the parasitic capacitance of the emission control transistor.
When the coupling current flows, the light-emitting element instantaneously emits light even though black display is being performed. As a result, since a black level rises, the contrast decreases, thus since this phenomenon is easily registered by the human eye, there is a direct association with deterioration of the quality of a display image.
That is, it is apparent from the inventor's examination that an important factor directly associated with decrease in the contrast at the time of black display is a leak current, which is generated due to a problem related to a circuit, not a leak current based on the physical characteristics of a TFT, which has been an issue in the related art.